(1) Field of the Invention
This present invention relates to ceramic materials to be used when hollow tubular ceramic members are to be insert-cast with a molten metal such as aluminum or cast iron. Particularly, the invention relates to ceramic materials to be used for insertcasting when inner surfaces of exhaust ports in gasoline engines, diesel engines or the like are to be lined. Further, the invention relates to ceramic port liners used to line inner surfaces of exhaust ports in gasoline engines, diesel engines or the like.
(2) Related Art Statement
Environmental pollution with exhaust gases from automobiles has become a serious social problem, and poisonous substances contained therein have mainly been removed by catalysts. From the standpoint of resources and costs, consumption amounts of noble metals such as Pt or Rh used as these catalysts have been demanded to be reduced. In addition, recently increasing 4 valve type engines have a problem in that purifying performance of catalysts is deteriorated due to reduction in temperatures of exhaust gases. As one technique for solving these problems, it has conventionally been proposed that temperatures of exhaust gases are raised by a thermally insulating action of ceramic port liners which are lined to inner surfaces of engine exhaust ports. In general, in order to fit ceramic port liners to engine exhaust ports, the ceramic port liners are insert-cast with a metal such as aluminum simultaneously when engine cylinder heads are produced. However, great compression stresses act due to shrinkage through solidification of the molten metal. Thus, there is a problem in that when stresses locally concentrate upon even a high strength ceramic material, the ceramic portion is broken.
On the other hand, Japanese patent publication Nos. 46; 26,330, 51-16,168, 60-5,544 and 56-7,996 propose ceramic materials used for hollow tubular members to be insert-cast with a metal.
Japanese patent publication No. 46-26,330 relates to an invention in which a ceramic member made of high strength alumina-based or silicon carbide based ceramic material is made porous at its surface portion to withstand shrinkage of a metal during cooling. However, such a composite material is in a yielded state in which fine cracks are latently present. Consequently, if such a composite material is used at a portion undergoing vigorous vibration, in such as in an engine, it is feared that the cracks proceed to conspicuously deteriorate durability.
Japanese patent publication No. 51-16,168 relates to an invention for producing heat insulating cast products having a low elastic modulus. The cast products are made of flexible ceramic semi-products consisting of a refractory filler and alumina cement, and their values of elastic modulus are 950 kg/mm.sup.2 at the minimum. Thus, they may be broken due to compression stresses during insert-casting.
Japanese patent publication No. 60-5,544 is aimed at low thermal expansion (thermal shock resistance) and high strength by adding kaolin and magnesium silicate to aluminum titanate. However, the minimum elastic modulus is 8.times.10.sup.3 kgf/mm.sup.2. Thus, breakage due to compression stress during insert-casting cannot completely be prevented by this method.
Japanese patent publication No. 56-7,996 is to afford low thermal expansion and high strength to aluminum titanate by adding SiO.sub.2 and ZrO.sub.2 thereto. Growth of crystalline grains of aluminum titanate is restrained by these additives. However, this method also has the above-mentioned shortcoming possessed by the prior art techniques.
In addition to the above-mentioned prior art methods, many attempts have been made to mitigate compression stresses due to a molten metal during shrinkage in solidifying by coating the outer periphery of a ceramic member with a layer of asbesto, ceramic fibers made of alumina or the like to be interposed as a buffer member between the metal and the ceramic member by utilizing an elastic force possessed by the layer (for instance, Japanese patent application Laid-open Nos. 52-121,114 and 59-175,693). However, since the outer periphery of the ceramic member is coated with the ceramic fibers, a final product is more costy due to an increased number of steps. Furthermore, this method has a serious problem in that if elasticity of the ceramic fiber layer is lost due to high temperatures and vibrations during engine operation, the port liner is peeled off from the metal member.
Further, Japanese patent application Laid-open No. 60-169,655 proposes hollow tubular ceramic members in which at least one high pressure-proof reinforcement is provided around the outer peripheral surface thereof. Since such ceramic members use tialite (aluminum titanate) and zirconia as ceramic bodies and the reinforcement, respectively, differences in thermal expansion and Young's modulus between them are so great that the ceramic members cannot withstand thermal shock and stress and are broken when insert-cast with aluminum.
Moreover, many attempts have also been proposed to prevent such breakage by making port liners themselves of a high strength material. However, since an extremely great stress is concentrated upon a depressed portion such as a central portion of a two barrel type port liner used in a four valve type engine, in which the outer surface is depressed inside, cracking cannot completely be prevented there.